The improved cutter is especially adapted for use on a lawn mower of the type having a cutter blade rotatable about an upright axis, whereby the cutting action is initiated at the periphery or outermost radius of the rotating blade cutting edge as the blade is moved in a plane generally perpendicular to the orientation of the objects being cut; although, the principles of the invention have application to other cutters, particularly to those for cutting brush and like vegetation, for example.
The cutter blade in a conventional or typical rotary lawn mower has diametrically opposed arms, each provided with a sharpened leading edge and a trailing edge which is shaped to create a "fan action" for lifting the grass and blowing cut grass through an outlet. The leading edge is generally parallel to a radius through the center or axis of rotation and essentially midway between the leading and trailing edges and thus presents itself as a shear or impact edge to the vegetation encountered.
The present invention involves a blade having a slicing rather than a direct shearing action, and this is achieved by providing on a blade having at least a pair of diametrically opposed arms a slicing edge at each of the radially outermost extremities of the blade. Each slicing edge is directed rearwardly and also radially outwardly from the leading edge toward the trailing edge. The slicing edge is especially fashioned or configured so as to create angles as defined by the tangent to the arc of rotation and the tangent to the slicing edge at the point of contact with an object being sliced, of zero degrees to eighty degrees. Further the slicing edge is fashioned or configured to occur as a minimum within the radial distance from the outermost extremities of the blade an amount equal to the maximum exposure of the rotating blade to uncut grass that is encountered with each revolution of the blade as the mower is moved through the grass in a plane parallel to the plane of blade rotation. This radial distance can be defined by the following formula: ##EQU1## Where: V=Travel velocity of mower in feet per second (f.p.s.)
N=Number of cutting ends on blade PA1 R=Revolutions per minute (r.p.m.) of rotating blade
Use of this formula will demonstrate that in the typical rotary lawn mower having a forward velocity of two and one-half to three f.p.s. and a blade rotating at 2500 to 3000 r.p.m., the maximum exposure or radial distance of a two-edged blade to uncut grass is about one-half inch. According to the present invention, the slicing edge is fashioned or configured throughout this radial distance as a minimum in order to function as a slicing edge with or without an accompanying conventional or near conventional cutting edge.
In a modified form of blade, the slicing edge may be extended rearwardly and outwardly as a "flare" continuation of the basic slicing edge. In a further modification of the invention, each outer blade portion having the slicing edge is offset downwardly from the plane of rotation of the main body of the blade. Another modification provides a cutter blade having four arms spaced apart at ninety degrees about the axis of rotation, each arm having a slicing edge as described.
In one prior art example of modified cutter blade, each leading cutting edge has a slight rearward slope from the conventional position of a leading shearing edge but these slightly sloping edges are by no means slicing edges but function primarily as direct shearing edges and as an aid for expulsion of cut vegetation from the rotating blade. See the U.S. Pat. No. to Hill, 3,315,451.
Advantages of the invention are improved cutting action resulting in cleaner cutting and less stress on the vegetation; less torque load on the prime mover; less fluctuation in speed of rotation of the blade; and less impact stress on the blade in the event a hard object is encountered, resulting in longer edge life and longer intervals between edge sharpening.
Further features and advantages will appear as preferred embodiments of the invention are disclosed in detail in the ensuing description and accompanying sheets of drawings.